The present invention relates to the art of medical diagnostic equipment. It finds particular application in conjunction with measuring xenon concentration in end-tidal gases. However, it is to be appreciated that the invention will also find application in conjunction with measuring concentrations of other gases.
Many medical diagnostic techniques call for a measurement of an absorbed gas in the patient's blood. Arterial blood gas concentration are in equilibrium with lung gases that are in intimate contact with the alveoles. These lung gases, denoted as end-tidal gases, are found at the end of the tide of the exhaled breath. By measuring the concentration of the gas in question in these last bits of the exhaled gas, the concentration of the gas in the blood can be determined.
The xenon concentration in the end-tidal gas can be measured by placing a thermoconductivity detector in the exhalation line. See U.S. Pat. No. 4,622,976, issued Nov. 18, 1986 to G. M. Tempe, et al., which measures xenon concentration, carbon dioxide concentration, or the like. Because the thermoconductivity detector will indicate xenon concentration continuously, it was necessary to determine which reading represents the end-tidal gases.
In one end tidal identifying technique, a chamber or reservoir was formed in the exhaled gas line leading from the breathing mask to hold exhaled gas. A mechanical system determined when the patient started to inhale fresh gas from a supply line. The thermoconductivity of the gas retained in the exhale chamber was measured. See for example U.S. Pat. No. 4,535,780, issued Aug. 20, 1985, to Gur, et al. One of the problems of this technique is that the apparatus is expensive. Another problem is that the mechanical means for determining the changeover point between inhaling and exhaling tends to be relatively unreliable. Another disadvantage is that exhaled gases intermix in the chamber diluting the end-tidal gases with earlier exhaled gases.
Another technique for determining the end-tidal gas was to monitor the xenon concentration continuously and assume that the xenon concentration minima were attributable to the end-tidal gases. However, when the patient began breathing rapidly, the minimum xenon concentrations did not correspond to end-tidal gas. Further, after the first few patient breaths, the magnitudes of the minima increased sufficiently close to the breathing gas xenon concentration that they were difficult to identify. Commonly, the end-tidal gas minimum values became substantially indistinguishable from the exhaled xenon concentration in the rest of the exhaled gas after the first minute of a five to seven minute xenon protocol.
The present invention provides a technique for determining end-tidal gases which overcomes the above referenced problems and others.